Today, plastic containers in different shapes and sizes—for example, bottles—are often made with a process comprising a blow moulding stage wherein the thermoplastic material, possibly in the form of a preform (i.e. a smaller, pre-moulded workpiece that is normally cylindrical in shape), is blow moulded in a heated mould to create the final container.
To date, two general known systems are used to heat the mould: the first uses film electric resistors placed near the inside surface of the mould, and the second uses a heating element consisting of a hot fluid that circulates in a coil placed in the mould.
Although the heating system with film electric resistors (described, for example, in U.S. Pat. Nos. 5,007,818 and 5,234,637) has excellent temperature modulation properties, it has a poor heating value. As a result, this type of heating system must be placed as near as possible to the surface of the mould that comes into contact with the material to be moulded. The drawback of this requirement is that it is necessary to have several moulds (i.e. one for each type of container or bottle to be moulded) equipped with a blanket of film electric resistors near the work surface; obviously, this increases costs.
The heating system with the circulating hot fluid has greater thermic inertia than the other system, although it has a good heating value. Furthermore, in today's rotary moulding machines, it presents significant problems in terms of assuring the watertightness of the location where the fixed and the rotary parts come into contact and of the ducts that convey the hot fluid, generally oil, to the mould, which opens to house the preform and closes to unload the finished product at high rates. These requirements add bulk to the machine and, thus, mean larger dimensions and higher costs. Another drawback is the poor thermal efficiency due to the dissipation of the thermal power in the rotary device since it is not possible to prevent the ducts for delivering the hot fluid from being far away from those for removing the cold fluid.
Today, thus, there is a need for a mould heating system for use in rotary blow moulding machines that is simple and effective.